So I have an RB20 in an S13, the motor is all stock. I have a FMIC, Walbro 255, tomei adjustable FPR, and a wideband gauge. After the FPR install the rich condition got better, especially under load. My main issue is at idle, mainly when I'm about to pull into a bay for emissions testing, it starts to run super rich, like 10:1. The idle starts going up and down too. I found that if I pop the hood and leave it up it seems to get better. It seems like the ECU wants to dump fuel when the underhood temp gets too hot. No overheating or cooling problems and the CTS is fairly new, I used a Z32tt one. I sometimes wish this was an OBDII car so I could actually see what the ECU is seeing.

You can but you need to wire in the consult port and get a consult cable and something like nissan data scan.

I Think I have solved my issue. I was reading through the FSM about idle speed and fuel mixture. Apparently, the oxygen sensor also plays a role in the idle speed. I have never been able to get a good solid idle, it has always been right at 900. I hooked up my meter to the O2 signal wire and it stayed around .325 V. When I held the RPMs at 2000, the voltage started creeping up until it started to switch lean to rich like it should. When I let it idle again it would continue for about a minute, then back to .325 ish volts. The O2 sensor needs to be around 600 degrees to keep producing signal, it was cooling down too much at idle, then run rich. I checked the heater wires at the O2 and was OK. I had power but no ground. I added a temporary ground and voila It now idles at 14.8 AFR. Then I set the idle to around 600 per the FSM, it stays there once warmed up. I checked the timing and was off by 5 degrees. Went for a drive before going to emissions, My HC went from 251 PPM down to 56! Right where the FSM says it should be. Hopefully someone can save some time and frustration from this.